feat(ui): collapsible date filter with sort + filter toggle on person row

Move sort button and filter toggle to the person row, matching the
document search page pattern (sort + filter + count inline). Date
range inputs are now a collapsible section behind the filter toggle,
using slide transition and the same grid layout as the document
search advanced filters. Fix date input padding (add px-3).

Refs: #179

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/briefwechsel/+page.svelte

@@ -28,6 +28,7 @@ $effect(() => {
 });
 
 const isSinglePerson = $derived(!!senderId && !receiverId);
+let showAdvanced = $state(false);
 
 const RECENT_STORAGE_KEY = 'korrespondenz_recent_persons';
 const MAX_RECENT = 5;
@@ -107,20 +108,23 @@ function selectPerson(id: string) {
 				bind:receiverId={receiverId}
 				initialSenderName={data.initialValues.senderName}
 				initialReceiverName={data.initialValues.receiverName}
-				onapplyFilters={applyFilters}
-				onswapPersons={swapPersons}
-			/>
-
-			<CorrespondenzFilterControls
-				senderId={senderId}
-				bind:fromDate={fromDate}
-				bind:toDate={toDate}
-				bind:sortDir={sortDir}
+				sortDir={sortDir}
+				showAdvanced={showAdvanced}
 				documentCount={data.documents.length}
 				onapplyFilters={applyFilters}
+				onswapPersons={swapPersons}
 				ontoggleSort={toggleSort}
+				ontoggleAdvanced={() => (showAdvanced = !showAdvanced)}
 			/>
 
+			{#if showAdvanced}
+				<CorrespondenzFilterControls
+					bind:fromDate={fromDate}
+					bind:toDate={toDate}
+					onapplyFilters={applyFilters}
+				/>
+			{/if}
+
 			{#if isSinglePerson}
 				<SinglePersonHintBar
 					senderName={senderName}